IT IS A TRUE FACT THAT SUGRIVA MARRIED VALI'S WIDOW TARA AFTER THE DEATH OF VALI
SUGRIVA WAS NO LAKSHMANA WHO WOULD CONSIDER HIS 'BHABHI' AS HIS MOTHER.
it has been stated in valmiki ramayana by valmiki himself that after the death of vali,sugriva married his widow tara and during 4 months of rainy season he has lots of sexual enjoyment with his 'original' wife ruma and newly acquired tara.
when lakshmana had gone to sugriva to complained about the delay then sugriva terrified and sent tara who was having sex with her new husband sugriva and she protected sugriva by arguing with lakshmana about the powers of sex and lust,tara was half nude when she came to meet lakshmana and lakshmana bowed down his head in shame when he saw her.(can see this whole under kishkindha kand of www.valmikiramayana.net )
and
the kamban ramayana version of sugriva tara episode is wrong because the kamban wrote his version in 16th century while valmiki wrote his ramayana(which is the original one) in the era of lord rama while consulting with seetaji,lord rama and lakshmana as well.
it was said sage valmiki had got yogic powers and while he meditated every scene of ramayana ocurrs in his vision and he wrote it as exactly and precisely without any alteration and change (like sanjay in mahabharatha saw the battle and narrated it to dhritrashtra)
BUT
KAMBAN CONSIDERED MORALITY AND INFUSED THE FICTIONAL ELEMENT OF SUGRIVA TREATING TARA AS MOTHER FIGURE AND KAMBAN ALTERED THE FACTS AND EVENTS ESPECIALLY INCASE OF SUGRIVA AND TARA.
IT WAS ALSO STATED IN RAMCHARIT MANAS BY TULSIDAS IN A VERSE THAT
SUGRIVA AND VIBHISHANA REPEATED THE SAME SIN OF VALI(MARRYING BROTHERS' WIFE) BUT LORD RAMA FORGAVE THEM AND DID NOT GAVE ANY PUNISHMENT TO THEM THAT WAS LORD RAMA'S LOVE FOR HIS DEVOTEES
HERE IS THAT LINK
THIS IS THE TRUTH OF SUGRIVA AND VIBHISHANA THEY WERE NOT AS PIOUS AND PURE AS LAKSHMANJI.